Chiral Pathways and Periodic Decay in <i>cis</i>-Azobenzene Photodynamics

Azobenzenes are candidates for efficient, photochemically triggered switching in devices of molecular size. The <i>cis</i>-azobenzene isomer is inherently chiral because of its helicity. Applying OM2/MRCI surface-hopping molecular dynamics simulations, we analyze chiral photoisomerization pathways in <i>cis</i>-azobenzene and correlate oscillatory features in the population decay to modes that trigger motion toward and from the S₁/S₀ crossing region

Nonadiabatic Decay Dynamics of a Benzylidene Malononitrile

The photoinduced nonadiabatic decay dynamics of 2-[4-(dimethylamino)­benzylidene]­malononitrile (DMN) in the gas phase is investigated at the semiempirical OM2/MRCI level using surface hopping simulations. A lifetime of 1.2 ps is predicted for the S₁ state, in accordance with experimental observation. The dominant reaction coordinate is found to be the twisting around the C7C8 double bond accompanied by pronounced pyramidalization at the C8 atom. Motion along this coordinate leads to the lowest-energy conical intersection (CI_01α). Several other S₀/S₁ conical intersections have also been located by full optimization but play no role in the dynamics. The time-resolved fluorescence spectrum of DMN is simulated by computing emission energies and oscillator strengths along the trajectories. It compares well with the experimental spectrum. The use of different active spaces in the OM2/MRCI calculations yields similar results and thus demonstrates their internal consistency

Interfacial States in Donor–Acceptor Organic Heterojunctions: Computational Insights into Thiophene-Oligomer/Fullerene Junctions

Donor–acceptor heterojunctions composed of thiophene oligomers and C₆₀ fullerene were investigated with computational methods. Benchmark calculations were performed with time-dependent density functional theory. The effects of varying the density functional, the number of oligomers, the intermolecular distance, the medium polarization, and the chemical functionalization of the monomers were analyzed. The results are presented in terms of diagrams where the electronic states are classified as locally excited states, charge-transfer states, and delocalized states. The effects of each option for computational simulations of realistic heterojunctions employed in photovoltaic devices are evaluated and discussed

Femtosecond Spectroscopy of Calcium DipicolinateA Major Component of Bacterial Spores

Bacterial spores are rich in calcium dipicolinate (CaDPA). The role of this compound in the high UV resistance of spore DNA and their unique DNA photochemistry is not yet clarified. Here, the photophysical properties of CaDPA dissolved in water are studied by means of steady-state and time-resolved spectroscopy as well as quantum chemistry. Upon 255 nm excitation, a fluorescence emission with a yield of 1.7 × 10^–5 is detected. This low yield is in line with a measured fluorescence lifetime of 110 fs. Transient absorption experiments point to further transitions with time constants of 92 ps and 6.8 μs. The microsecond time constant is assigned to the decay of a triplet state. The yield of this state is close to unity. With the aid of quantum chemistry (TD-DFT, DFT-MRCI), the following transitions are identified. The primarily excited ¹<i>ππ</i>* state depletes within 110 fs. The depletion results in the population of an energetically close lying ¹<i>nπ</i>* state. An El-Sayed allowed intersystem crossing process with a time constant of 92 ps ensues. Implications of these findings on the interaction between photoexcited CaDPA and spore DNA are discussed

Relationship between Excited State Lifetime and Isomerization Quantum Yield in Animal Rhodopsins: Beyond the One-Dimensional Landau–Zener Model

We show that the speed of the chromophore photoisomerization of animal rhodopsins is not a relevant control knob for their light sensitivity. This result is at odds with the momentum-driven tunnelling rationale (i.e., assuming a one-dimensional Landau–Zener model for the decay: Zener, C. Non-Adiabatic Crossing of Energy Levels. <i>Proc. R. Soc. London, Ser. A</i> <b>1932,</b> 137 (833), 696–702) holding that a faster nuclear motion through the conical intersection translates into a higher quantum yield and, thus, light sensitivity. Instead, a model based on the phase-matching of specific excited state vibrational modes should be considered. Using extensive semiclassical hybrid quantum mechanics/molecular mechanics trajectory computations to simulate the photoisomerization of three animal rhodopsin models (visual rhodopsin, squid rhodopsin and human melanopsin), we also demonstrate that phase-matching between three different modes (the reactive carbon and hydrogen twisting coordinates and the bond length alternation mode) is required to achieve high quantum yields. In fact, such “phase-matching” mechanism explains the computational results and provides a tool for the prediction of the photoisomerization outcome in retinal proteins